in my undergraduate chemistry lab we synthesized the substituted benzene on the left in a Friedel-Crafts reaction. As a next step we're supposed to reflux for 1.5h at 90°C with concentrated sulfuric acid and obtain the product on the right. A bicyclical product.

We have to come up with a plausible mechanism for the ring closure reaction and I am unsure that what I came up with makes sense or not.

Thank you!

enter image description here

edit: this is the FC reaction we did in the first step: enter image description here

The Steps for this reaction are as follows:
To a Solution of AlCl3 in abs. DCM the following reagents were added:
-3-chloropropionic chloride.
The reaction mixture was stirred at rt for 1h.
Then added to a mixture of 1:1 conc. HCl and ice water.
Then extracted with DCM in a separatory funnel.
Washed twice with a sat. NaCl solution.
The organic phases were dried with MgSO4.
The DCM was then removed in the rotary evaporator.
A yellow-orange Solution was left at the end.
It was left to crystallize over night.

Now this is where the second part happens, the H2SO4-cyclisation reaction.


Some comments suggested that there might be a hydrolyzed intermediate with a displaced chloride. Here is my take on how that would look like: enter image description here

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    $\begingroup$ Are you sure you made the starting material shown by the F-C reaction? What went into it? $\endgroup$ – Waylander Nov 23 '19 at 17:15
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    $\begingroup$ Yes, I'm reasonably sure that it's the product. We began with para-xylene, 3-Chloropropionyl chloride, AlCl3 as the catalyst. (DCM was the solvent) $\endgroup$ – Michael Nov 23 '19 at 18:04
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    $\begingroup$ OK, that looks to be the correct intermediate. It's just that the H2SO4 cyclisation conditions would be far more appropriate for cyclising a carboxylic acid. What solvent is the cyclisation done in? $\endgroup$ – Waylander Nov 23 '19 at 18:31
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    $\begingroup$ H2SO4 has to be added to the starting material which is crystalline. After the reflux step, there's an aqueous workup and an extraction with ether. Thank you very much for taking the time to answer my question. $\endgroup$ – Michael Nov 23 '19 at 18:48
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    $\begingroup$ You should investigate the Nazarov reaction. $\endgroup$ – user55119 Nov 23 '19 at 23:41

I would propose the mechanism illustrated below, which would seem plausible in concentrated sulfuric acid:Proposed electrophilic mechanism of ring closure

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  • $\begingroup$ I'm doubtful about the mechanism of your chloride elimination $\endgroup$ – Waylander Nov 24 '19 at 14:53
  • $\begingroup$ your mechanism is in line with the Nazarov Cyclization, interesting! $\endgroup$ – Michael Nov 24 '19 at 15:58
  • $\begingroup$ I think an allyl chloride with an electron donating OH group conjugated in the 3 position could likely solvolyze in concentrated sulfuric acid. Then the electrophilic substitution is intramolecular, therefore favorable. $\endgroup$ – iad22agp Nov 24 '19 at 17:11
  • $\begingroup$ I had the reaction today and this was more or less the accepted mechanism. Thanks! $\endgroup$ – Michael Nov 27 '19 at 16:34

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